This invention relates generally to calcium carbonate pigments for use in papermaking, and more specifically relates to a calcium carbonate pigment having acid resistant properties.
Calcium carbonate pigments have found increasing application as fillers (and also as coating pigments) in the production of paper and paperboard products. This utility arises by virtue of their excellent optical and physical properties when used for such purposes. In general, however, such calcium carbonate pigments have principally been utilized in papermaking operations based upon neutral or alkaline chemistry. Papermaking operations based upon use of ground wood, however, normally are conducted at an acid pH, and under these conditions calcium carbonate cannot normally be used because of its ready reactability.
It has heretofore been recognized that such calcium carbonate pigments, including especially precipitated calcium carbonates ("PCC's"), i.e. carbonates commonly produced by reaction of a slurry of calcium hydroxide with gaseous carbon dioxide, may by several known techniques be provided with a protective surface coating, rendering the pigments acid resistant. Several such techniques have been based upon precipitation of silica or a metal silicate upon the surface of the calcium carbonate pigment. One way in which this can be accomplished is by reacting water glass (sodium silicate solution) with sulfuric acid, so that the silica will precipitate on the particulate calcium carbonate pigments present in the reaction slurry. However, evaluation of this technique has shown that the resultant modified calcium carbonates, e.g. surface modified PCC's, have only quite limited resistance to acidity.
In another known process a PCC slurry is intermixed with sodium silicate solution, and then the latter is subjected to a two-step carbonation with gaseous carbon dioxide, namely to a first carbonation step at approximately 80.degree. C. and then (following cooling of the slurry to around 25.degree. C.) to a second carbonation step with gaseous CO.sub.2. This results in precipitation of silica as aforementioned.
More recently, in European application No. 89650222.4 (publication number 0356406), the applicants Tokarz et al. describe a process wherein a previously prepared PCC slurry is intermixed with a solution of a zinc compound and a solution of a silicate-containing substance. According to the invention, a slurry of CaCO.sub.3 filler particles is mixed simultaneously with a solution of a zinc compound and a solution of a silica-containing substance which preferably is sodium water glass. The zinc compound preferably is zinc chloride or zinc oxide. The two solutions are added at a temperature of 70.degree.-95.degree. C., preferably 85.degree.-90.degree. C. especially preferred 90.degree. C. Furthermore, the addition is carried out pH 8-11, preferably 9-10, especially preferred 9.2-9.4. The added amounts of coating agent, i.e. of a silica-containing substance, preferably water glass, and of a zinc compound, preferably zinc chloride, are 300-400 parts by weight SiO.sub.2 and 40-60 parts by weight Zn. The SiO.sub.2 :Zn ratio is 7.5:1 to 6.6:1, which corresponds to pH 8-11.
The product provided from this process has excellent acid resistant properties. However, it may be noted that as a practical matter the use of zinc chloride or zinc oxide is a very costly and inefficient way to produce the said product. Thus the commercial attractiveness of such a process is limited by its costs.
Parenthetically it may be noted that the broad concept of treating inorganic mineral products, including calcium carbonates, with a metal silicate has been previously known in the art as shown, for example, in Williams, U.S. Pat. No. 4,167,423. Example III of that patent, by way of illustration, shows preparation of a calcium silicate treated carbonate filler by reaction of hydrochloric acid with a sodium metasilicate nonhydrate. In other Examples of Williams, different water soluble salts were employed including zinc chloride. Williams' objective is actually to produce a product which is silane reactive. It will be appreciated that the method of Williams is not used in conjunction with a carbonation process as is commonly practiced in the preparation of especially PCC's, and teachings such as those of Williams fail to recognize the critical conditions that are necessary to produce acid resistant calcium carbonate pigments, including in a directly usable slurry form.